Dielectric composition



April 1943. A. w. THOMPSO.N 2,316,299

DIELECTRIC COMPOS ITIONS Filed Jan. 19, 1940 lb 2 1 16 2 11 16 w Patented Apr. 13, 1943' DIELECTRIC COMPOSITION Archibald Walter Thompson, Bromley, England I Application January 19, 1940, Serial No. 314,735 In Great BritainJanuary 25, 1939 1 Claim.

This invention is for improvements in or relating to dielectric compositions, particularly dielectric compositions for high voltage electric cables.

In the past it has been the practice of cable manufacturers to impregnate their paper dielectric insulated cables with mixtures containing for example mineral oil, fatty oils, petroleum Jelly and rosin. 'Initially, rosin was added simply in order to impart to the dielectric composition the desired mechanical properties.

With the advent of high voltages and the subsequent progress in the manufacture of cables suitable for carrying such voltages, attention has been given to improving the electrical properties of the dielectrics employed. Thus, it was found that improvements in the refining of mineral oils brought about a similar improvement in the composite cable dielectric formed by impregnating for example paper with the oil. Unfortunately, however, such improvement in the mineral oil was lost when the oil 'was compounded with rosin.

unsaturation, assists the prevention of gas for-' mation in the oil when the dielectric is subjected to high electric stress, and also prevents migra tion of the dielectric compound in the cable by increasing to a large extent the viscosity of the composition beyond that of the mineral oil.

One object of this invention is to provide a rosin-oil dielectric composition which shows a great improvement over previously known compositions, and furthermore to provide means whereby low grade rosin can be utilised in such a composition. Further objects are to provide a new or improved method of refining colophony, or low grade rosin, thereby to produce a material admirably suited for admixing with a mineral oil in the production of a dielectric composition, and to provide a new rosin-like material for use as a dielectric component.

According to this invention, a composition for use as a dielectric medium comprises an oil-soluble high vacuum-refined rosin dissolved in a mineral oil. Preferably the proportion of rosin lies between about 5 and 45% by weight of the composition. A

understood distillation of the character of that obtaining in a film evaporator or its equivalent, such, for example, as that hereinafter described, in which the effect of hydrostatic head in-the still is eliminated. In such an apparatus, the rosin on reaching the boiling point is converted into vapour within a very short period of time, for example, of the order of a few seconds.

Distillation should be carried out so as to be completed at the lowest possible temperature and it is found that rosin so prepared is practically entirely free from residual terpenes and from rosin oxide. The aggregate proportion of these impurities present in the product is not more than about 0.2% by weight. Preferably, the vacuum distillation of the natural resin is carried out in such a way that some or all of the latent heat of evaporation of the resin is supplied through the walls of a still comprising a heat exchanger over which the resin is caused to flow.

The composition of the present invention provides a dielectric material which has dielectric properties not obtainable in any other way. The dielectric loss of such a composition is approximately of that of the rosin-mineral oil compositions hitherto used.

Preferably, the mineral oil forming part of the dielectric composition, is derived from a naphthenic base crude oil and preferably has a viscisity of 200 seconds Redwood at 140 F. or higher. Highly saturated praffin oils are bad solvents for rosin and are bad solvents for any rosin oxide which may be formed by the oxidation of the rosin in the dielectric composition during its use.

Aromatic hydrocarbons on the other hand are too easily oxidised and are found to be unsuitable in other respects. The so-called naphthenic oils derived from the so-called naphthenic crudeoils are believed to be ideal solvents for the purpose in view.

The invention also includes a dielectric medium comprising a foraminous material, e. g. paper,

impregnated with the composition hereinbefore set forth. The invention also comprises an electrical element, particularly an electric cable or a condenser, in which the dielectric medium com-- prises the composition hereinbefore set forth, and in particular electric'cablesinsulatedwith paper impregnated with the new composition.

A further feature of the invention consists genorally in a method of refining natural rosin or colophony by vacuum distillation conducted in a manner such that some or all of the latent heat of evaporation of the rosin is supplied through the walls of a still comprising a heat-exchanger over which the rosin is caused to flow.

The distillation of the colophony in accordance with this invention may be carried out with plant of the kind already in use for the vacuum distillation of other organic material but care must be exercised in both the design and operation to take into account the nature and properties of the colophony. In the accompanying drawing, there is diagrammatically illustrated a suitable form of plant which has been specially adapted for the purpose, Figure 1 being a sectional elevation of the main parts of the plant and Figure 2 a localised section on the line AA of Figure 1.

Referring to the drawing, the reference numeral l designates a still which is fed with melted colophony from a feed-line ii via a helical trough If. The trough i2 is secured to the inner wall l3 of a jacketed container l4 forming an extension of the still iii. The space between the walls i3 and I4 communicates through a pipe I5 with two condensers i6 and i8 connected by a pipe I1, the condenser l8 being in turn, connected through the line I! with a high efficiency vacuum pump (not shown) of any suitable,

-known design or better there is provided a series of steam jets backed by a water-flooded vacuum pump.

It is found that it is important to construct the plant of some metal not materially attacked by rosin, e. g. iron or steel cannot satisfactorily be used, not because the product of the distillation is necessarily spoilt by iron, but because in the still iron rosinate is formed, accompanied by considerable quantities of non-condensible gas which makes it impossible to maintain a suitable vacuum. Iron or steel plant gives an excessive quantity of residue, rich in iron (14% has been observed) and apart from damage to the product the rate of corrosion of steel or iron is so great as to make their use impracticable. Suitable materials are stainless steel, enameled iron or steel, or aluminium. The plant is preferably constructed of stainless steel but aluminium may be used if due regard is had to its inferior mechanical properties at elevated temperatures.

Within the still i0 is a bundle of vertical tubes secured at each end in a liquid-tight manner in plates 2|, 22- supported in the still between pairs of flanges 23, 24 respectively. The bundle of tubes 20 is heated as by means of a vapour which condenses on the tubes or by means of a fluid which circulates around the outside of the tubes. Immediately beneath the lower end of the bundle 20 is a mechanically driven impeller 25 arranged with a suitable baffie 26 in such manner that it will force liquid upwards through the central tubes and draw liquid downwards through the outer tubes. The impeller drive shaft is indicated at 21. Above the tubes 20 the container increases in size and is of sufficient length to contain any froth that may form during distillation. As already stated, this upper part of the container is double-walled and is arranged to allow the vapours, after they leave the froth chamber, to pass downwardly between the inner and outer walls 13 and H. A spray:-

' trap or baffle |3a is arranged at the outlet end of the froth chamber to prevent anyraw material being carried over into the condenser system.

All the passages between the still and the vacuum pump are of large dimensions and are free of constrictions such as would cause an avoidable increase in absolute pressure between the still and pump.

No receivers are provided for the condensate from the condensers I6 and I8 but, in each case,

a heated, continuously acting pump 3|, 4| respectively, sealed by the condensate itself (received in the chambers 30, 40 respectively) is provided. Receivers may, however, be used if desired.

The method of operation is to boil the heating fluidwhich in this case is diphenyl oxide-under a constant predetermined pressure corresponding to a temperature of about 300 C. to 400 C. and to condense the vapour around the outside of the tubes 20. The condensed circulating fluid is returned for reheating. The plant is evacuated and a charge of melted natural rosin (temperature say 160 C.) is sucked into the still in. The incoming melted rosin is directed on to the inclined trough l2 and passes down the helix in a thin film to the still; in so doing the resin is effectively degasified. The initial charge is sumcient just to cover the ends of the tubes 20 and the level is kept substantially constant during distillation by the addition at intervals of fresh material.

As soon as the charge has been added to the still the impeller 25 is started and the melted rosin is continuously circulated through the tubes 20, absorbing heat from the diphenyl oxide vapour which condenses on the outside of the tubes 20 before returning for reheating. The velocity of the resin through the tubes is about 5 feet per second and should not be less than 3 feet per second.

In order to avoid cracking of the material, the temperatures of the material in the still should not exceed 300 C. and should preferably be about 225 C. to 235 C. which is sufiicient for distillation if the corresponding pressure of about 3 mm. of mercury can be maintained.

When the material in the still is substantially all residue, distillation and the supply of new material is stopped temporarily and recommenced when the residue has been run out into the previously evacuated receiver 28.

From the still the vapours pass vertically downwards through the first condenser i6 where they are condensed and cooled to a temperature of about C. and from there any remaining vapours are passed to the second condenser ll where'they are cooled to about 20 C. Condensate is removed via the condensate pumps ii and 4|, that removed from the firstcondenser constituting the refined rosin of the invention. Any vapours still remaining are passed to the vacuum pump which maintains a pressure of about 3 mm. of mercury throughout the system. This pressure should be as low as possible and never above 40 mm. of mercury and preferably not above 10 mm.

The following is an example of a dielectric composition according to the present invention:

Colophony obtained by the preferred method as above described is admixed with a refined mineral oil having a viscosity of about 200 seconds Redwood at F. and derived from the so-called naphthenic crude oil to form a solution. The composition may contain from about 5% to about 45% by weight of the refined rosin.

The mineral oil may advantageously receive a treatment with an adsorptive material, e. g.

activated clay or carbon, immediately before compounding with the resin. By this means, there is obtained a considerable improvement in the electrical properties of the oil which would not persist in the absence of the refined rosin. Apparently, the refined rosin employed in accordance with the present invention exerts a stabilising efiect on the treated mineral oil.

It is preferable when forming a solution of the rosin and the mineral oil to heat them together for a period not less than 30 minutes at a temperature of approximately 120 C. to 150 C. whilst agitating the mixture in order to ensure that the solution shall be stabilised. This precaution although desirable is not essential and the procedure adopted may vary with the nature of the oil, but it is a practice to be recommended provided that the oil itself is not such as will undergo severe oxidation during the treatment.

The dielectric composition thus prepared has a very low dielectric loss, a very high specific dielectric resistivity, a high resistance to oxidation, a high resistance to the formation of gas under electrical stress and shows a very high resistance to the formation of hydrocarbon polymers of high molecular weight known commercially as compound X or cheese.

The application of the dielectric composition naturally depends upon the particular type of electrical equipment with which it is to be used. Thus, in the case of a high voltage electric cable, any normal procedure may be followed. For example, paper in strip or sheet form may be first lapped on to the conductor, and then impregnated with the composition above described prior to the application of the usual lead-sheathing. The dielectric thus prepared has a very high thermal stability, very low power losses and shows better ageing characteristics than dielectrics hitherto known.

The use of the dielectric composition of the present invention reduces the power loss in the cable dielectric and thereby materially reduces the risk of break down of the cable whilst at the same time it renders possible the manufacture of cables for use on voltages higher than those at present in commercial usage, and furthermore, the reduction in bulk of the conductor elements for a given load. Thus, the important advantages of substantially improved safety-factor and economy are jointly obtained.

The outstanding advantages attained by the present invention are illustrated by the followsitions incorporating ordinary rosin, with a view to ascertaining the power factor and specific insulation resistance at different temperatures.

Power factor S. I. R.

:L 80 C. .00153..... 80 C. 16.2X10S2 per cm. 52BX10: per cm. l070 li2 per cm C. 67.0Xl0m per c111 2900x10 9 per mm. 2675x16 9 per cm. 562x10: per cm. 6 233.0 10m per cm. 20 C 00030.-.. 20 C. 2675x10 9 per cm I) 80 C 00371... 80 C. 4.1 m per cm 60 C 00076... 60 C. 20.0Xl0 Sl per cm C 00071..... 20 C. l070 l0i2 per cm.

In the above:

A is the oil as received, already well refined.

B is the oil immediately after subjecting to a supplementary refining process as hereinbefore described. In this condition-the oil would not remain stable, and would revert in the course of a few hours to A.

C is B+20% of the new refined rosin. The rosin now has the efiect of stabilising the compound and the tests indicated will remain indefinitely.

D is the same oil as B with the addition of 20% of ordinary unrefined rosin of first-class quality. The rosin used should have been about the best which is available on the commercial market.

As the power factor is a measure of energy lost in the dielectric, it is clear'that the sample C is without doubt greatly superior to the known type of composition D. Thus, whereas in the case of C the power factor is of substantially the same order of magnitude between the temperature range of 20-80 C., the sample D showed a five-fold power factor increase over the same range and, moreover, even at 20 C. the power factor for D was nearly double the corresponding figure for C at C. Similarly, but in an inverse sense, the specific insulation resistances show a comparison in favour of composition C.

I claim:

A new product of manufacture in the form of a dielectric composition comprising a naphthenic base mineral oil having a viscosity of at least 200 seconds Redwood at F. and dissolved therein a refined rosin containing a total of residual terpenes and rosin oxide that is substantially less than that of the impure colophony from which it is produced, and having substantially the same optical activity as said impure colophony.

ARCHIBALD WALTER THOMPSON. 

